A printed circuit board surface typically comprises 10-40-micron-thick copper features, commonly formed by subtractive patterning, on a polymer substrate such as FR-4. The topography of this surface presents several problems for the formation of embedded capacitors and resistors.
For screen printed polymer thick film resistors, the non-planar surface interferes with the squeegee motion, resulting in variability and thus inferior resistor tolerances. In addition, squeegee downward deflection between the two copper terminations of a resistor varies according to the length of the resistor—greater deflection occurs with longer resistors—which results in a non-linear dependence of resistance on length. This non-linearity also contributes to poor resistor tolerances when a range of resistor values is being printed. The problems become more severe for thicker conductor requirements. Finally, even the FR-4 surface on which a resistor body is printed is not smooth, but has a roughness that ranges from 3 to 12 microns, which is the impression left by the copper “tooth”. This roughness also compromises resistor tolerances, because it is on the order of one third or one half the resistor thickness.
For ceramic-filled polymer (CFP) embedded capacitors, the non-planar board surface makes it difficult to form a thin dielectric layer of uniform thickness. For low-flow embedded capacitance materials such as a CFP-coated foil distributed by Oak-Mitsui Technologies of Hoosick Falls, N.Y., the CFP resin does not fill the spaces between copper features, resulting in air pockets. For high-flow materials such as a CFP liquid resin distributed by Huntsman Corporation of Salt Lake City, Utah, the resin often fills the spaces between copper features, but unacceptably thin dielectric may result at the copper feature edges, risking shorts when the top electrode copper is applied. In addition, dielectric thickness over capacitor bottom electrodes will typically vary according to the particular board design. Dielectric thickness will typically vary even within one board design, the resin tending to planarize more readily over smaller copper features, resulting in a thinner dielectric than over large copper features.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the concepts.